Revert "[Reland #3] Use the SequenceManager in ScopedTaskEnvironment"

This reverts commit d1317fc30ce4e9ef08c5197c0f67767dfe585df4.

Reason for revert: SyncSchedulerImplTest is failing and this is the most likely culprit. More information in the linked bug.

Bug: 913946

Original change's description:
> [Reland #3] Use the SequenceManager in ScopedTaskEnvironment
> 
> A reland of https://crrev.com/c/1324391
> 
> This is necessary because we want content::TestBrowserThreadBundle to
> own a BrowserUIThreadScheduler, but that also owns a ScopedTaskEnvironment
> and you can't have two SequenceManagers on the same thread.
> 
> This patch allows ScopedTaskEnvironment to optionally work with an
> externally owned SequenceManager solving the problem.
> 
> This implements https://docs.google.com/document/d/1y08C6JQ9Yta3EQXzwIqqIIKHq9500WV6CWFZzZfDx7I/edit?usp=drivesdk,
> 
> We now have the ability to mock time on the UI and IO threads.
> 
> RE HttpServiceTest change HttpServiceTest.MultipleRequests was failing
> mysteriously on fuschia on the bots only (not locally). IO_MOCK_TIME seems
> to have fixed this.
> 
> 
> TBR=gab@chromium.org,fdoray@chromium.org,wez@chromium.org
> 
> Bug: 863341, 891670, 708584
> Change-Id: I95444a0a50f8b577c5fae62c12c8423c7e8f21d6
> Reviewed-on: https://chromium-review.googlesource.com/c/1361863
> Commit-Queue: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Alex Clarke <alexclarke@chromium.org>
> Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#615496}

TBR=wez@chromium.org,gab@chromium.org,fdoray@chromium.org,skyostil@chromium.org,alexclarke@chromium.org

Change-Id: If5ff0362758b5270eb3113d2fd6eeeb7facc37c3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 863341, 891670, 708584
Reviewed-on: https://chromium-review.googlesource.com/c/1371811
Reviewed-by: Friedrich Horschig [CET] <fhorschig@chromium.org>
Commit-Queue: Friedrich Horschig [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615520}
4 files changed